1. Field of the Invention
This invention relates to motion dependent video signal processing. More particularly, the invention relates to methods of deriving motion vectors representing motion between fields or frames of a video signal, and to video standards converters using such methods. Still more particularly, but not exclusively, the invention relates to the type of video standards converter generally known as and referred to herein as a down converter.
2. Description of the prior Art
Video standards converters are well known devices used to convert video signals from one standard to another, for example, from a 625 lines per frame, 50 fields per second standard to a 525 lines per frame, 60 fields per second standard. Video standards conversion cannot be achieved satisfactorily merely by using simple linear interpolation techniques, because of the temporal and vertical alias which is present in a video signal. Thus, simple linear interpolation produces unwanted artifacts in the resulting picture, in particular, the pictures are blurred vertically and judder temporally.
To reduce these problems it has been proposed that video standards converters should use adaptive techniques to switch the parameters of a linear interpolator in dependence on the degree of movement in the picture represented by the incoming video signal.
It has also been proposed, for example for the purpose of data reduction in video signal processing, to generate motion vectors from an incoming video signal by a block matching technique, in which the content of a search block in one field or frame is compared with the respective contents of a plurality of search blocks comprised in a search area in the following field or frame, to determine the minimum difference between the contents so compared, and hence the direction and distance of motion (if any) of the content of the original search block.
Our copending European patent specification EP-A2-0 395 275 (published Oct. 31, 1990) discloses a method of deriving motion vectors representing motion between successive fields or frames of a video signal, and in particular a method of selecting motion vectors appropriate to each pixel of an output field or frame. These applications also disclose a video standards converter using such methods, and particularly, but not exclusively, intended for use in the conversion of a high definition video signal (HDVS) having 1125 lines per frame, 60 fields per second, to 24 frames per second 35 mm film. The present invention is particularly concerned with modifying the algorithms used in the motion vector selection such that the amount of hardware required can be reduced, and also with widening the range of video standards converters in which the methods can be used to include down converters operable in real time.